Petroleum includes only crude oil, but in common usage it includes all liquid, gaseous, and solid hydrocarbons. Under surface pressure and temperature conditions, lighter hydrocarbons methane, ethane, propane and butane occur as gases, while pentane and heavier ones are in the form of liquids or solids. However, in an underground oil reservoir the proportions of gas, liquid, and solid depend on subsurface conditions and on the phase diagram of the petroleum mixture.
Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are less dense than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometers horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.
An oil well produces predominantly crude oil, with some natural gas dissolved in it. Because the pressure is lower at the surface than underground, some of the gas will come out of solution and be recovered (or burned) as associated gas or solution gas.
Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are less dense than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometers horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.
Commonly, wells are drilled into oil reservoirs to extract the crude oil, which will contain some amount of water. “Natural lift” production methods that rely on the natural reservoir pressure to force the oil to the surface are usually sufficient for a while after reservoirs are first tapped. In some reservoirs, such as in the Middle East, the natural pressure is sufficient over a long time. The natural pressure in most reservoirs, however, eventually dissipates. Then the oil must be extracted using “artificial lift” means. Over time, these “primary” methods become less effective and “secondary” production methods may be used. A common secondary method is “waterflood” or injection of additional water into the reservoir to increase pressure and force the oil to the drilled shaft or “wellbore.”
Eventually “tertiary” or “enhanced” oil recovery methods may be used to increase the oil's flow characteristics by injecting steam, carbon dioxide, hydraulic fracturing fluids, and/or other chemicals into the reservoir. In the United States, primary production methods account for less than 40 percent of the oil produced on a daily basis, secondary methods account for about half, and tertiary recovery the remaining 10 percent. Extracting bitumen from oil/tar sand and oil shale deposits requires mining the sand or shale and heating it in a vessel or retort, or using “in-situ” methods of injecting heated water into the deposit and then pumping out the oil-saturated liquid.
Vast quantities of water, in the form of steam, are used to liquefy and extract petroleum from tar sands, an unconventional oil reserve. When oil-eating bacteria biodegrade oil that has escaped to the surface, tar sands, such as the Athabasca oil sands in Canada, can result. Tar sands (more properly termed bituminous sands) are reservoirs of partially biodegraded oil still in the process of escaping and being biodegraded, but they contain so much migrating oil that, although most of it has escaped, vast amounts are still present—more than can be found in conventional oil reservoirs. Canada and Venezuela have the world's largest deposits of oil sands.
Thus, crude oil from many sources, both conventional and unconventional, contains amounts of water that can vary from a percentage of the extracted petroleum to a multiple of the total volume of extracted petroleum. This water, which is often in an emulsion with the extracted petroleum, must be separated from the oil before the oil can be further refined and used.
Various methods exist for dehydrating, or dewatering, crude oil. Thus, U.S. Reissue Pat. No. RE33999 is drawn to a method and apparatus for removing residual water from heavy crude oil. The apparatus comprises a distillation apparatus comprising a casing, an inlet for admitting liquid crude oil into the casing so as to establish a liquid surface in the casing, an outlet passage for discharging dehydrated liquid crude oil from the casing, a heater in the casing for maintaining the liquid oil at a distillation temperature for evaporating water and light hydrocarbons, a vapor outlet for discharging a mixture of water vapor and light hydrocarbons evaporated from the crude oil, a spray device above the surface of the liquid in the casing for spraying incoming crude oil onto and in heat exchange contact with the surface of the heated oil in the casing, whereby water and light hydrocarbon vapors are distilled from incoming crude oil upon contact with the surface of the heated liquid crude oil.
U.S. Pat. No. 1,559,036 is drawn to an electrolytic method for demulsifying oil and water emulsions, involving adding a chemical electrolyte to the emulsion, then passing an electrical current through the emulsion to separate water and oil phases.
U.S. Pat. No. 3,453,205 describes heating the crude oil to coalesce and evaporate the water, then skimming the oil into a separate compartment.
These basic methods of heating and/or distilling the water from the petroleum, and of the use of electrical current to resolve the oil-water emulsion (and combinations of these approaches) are still in use.
Additionally, as described by European Patent Publication EP 2231822, chemical methods of demulsifying oil-water emulsions and dewatering crude oil are also used. Dewatering in this fashion can be an expensive step in the process of upgrading crude oil for transportation and/or refining due to the slight differences in specific gravity between the oil and water, the expense of the chemical used, and the need for time to permit the separation to occur. Large separation vessels, for example, have been used to phase separate the water from the oil, with long residence times for the separation to take place.
European Patent Publication EP 0174399 describes the use of dehydrated and/or desalted by an aqueous formulation comprising (i) a demulsifier such as an alkylene oxide alkyl phenol-formaldehyde condensate (e.g., a polyethoxylated nonylphenol-formaldehyde condensate) and (ii) a deoiler to prevent oil entrainment into the aqueous phase such as a polyol e.g., ethylene glycol or polyethylene glycol having a molecular weight ranging from 106 to 4500.
U.S. Pat. No. 5,989,436 describes a method for dehydrating crude oil wherein an emulsion of water and oil is mixed in a stirring chamber together with a suitable amount of an emulsion breaker. The resulting mixture is subjected to separation in a two-phase separation tank to a oil-rich component and a water-rich component, the oil-rich component is then fed to a dehydrator of a high voltage charge type in which separation to a dehydrated heavy oil and a second water-rich component is effected, the two water-rich component fractions supplied from the two-phase separation tank and the dehydrator are mixed in a pipeline, the mixture thus obtained is admixed with an oil-in-water emulsion breaker and then, fed to a stirring chamber and finally separated to a second oil-containing component and a oil-free water layer in a second separation chamber.